High vacuum pump



00L 1955 R. B. LAWRANCE EI'AL 2,721,025

HIGH VACUUM PUMP Filed Dec. 1, 1953 2 Sheets-Sheet l Q a 2 10 a Q a zowso INVENTORS R ha d 5. [awrcmcz y GOF9 14. fie/Er ATTORNEY Oct. 18, 1955 R. B, LAWRANCE ETAL 2,721,025

HIGH VACUUM PUMP 2 Sheets-Sheet 2 Filed Dec. 1, 1955 FIG.

FIG

INVENTORS Bic/70rd 5. ZawF BY 6Of9 7 ATTORNEY United States Patent HIGH VACUUM PUMP Richard B. Lawr'ance and George A. Safer, Cambridge, Mass., assignors to National Research Corporation, Cambridge, Mass., a corporation of IVlassachusetts Application December 1, 1953, Serial No. 355,402

4 Claims. (Cl. 230-101) This invention relates to a high vacuum pump and more particularly to an improved high vacuum diffusion pump.

A principal object of the present invention is to provide an improved diffusion pump which is rugged in construction and cheap to manufacture.

Another object of the invention is to provide a diffusion pump of the above type which can be readily assembled and disassembled with accurate positioning of the parts thereof for optimum operating conditions.

Still another object of the present invention is to provide a diffusion pump of the above type which is capable of preventing losses of appreciable quantities of pump oil.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

Fig. l is a diagrammatic sectional View of one preferred embodiment of the invention, and

Fig. 1a is an exaggerated enlarged fragmentary view of a portion of Fig. l, and

Fig. lb is also an exaggerated enlarged fragmentary view of a portion of Fig. 1.

In general, the present invention is directed to improved vacuum diffusion pumps capable of high speed pumping operation and having blank off pressures on the order of from about lO mm. Hg absolute to about l mm. Hg absolute. In a preferred embodiment of the present invention, the diffusion pump comprises a pump body and a vapor jet assembly positioned within the pump body which defines a pumping chamber with the pump body. At the bottom of the pump body, there is provided a boiler for a diffusion pump oil, the bottom of the jet assembly being open to pump oil vapors generated in the boiler portion. A foreline is connected to the pump body adjacent an opening therein which is positioned below the lowest jet on the jet assembly near the bottom of the pumping chamber. A resilient perforated member is positioned below the foreline so that it bridges the space between the jet assembly and the pump body. The resilient perforated member serves a number of purposes. It acts to space the jet assembly concentrically with respect fo the pump body. It also provides a high impedance path for pump oil vapors tending to travel upwardly, from the boiler, to the pumping chamber. It also prevents excessive loss of oil. Lastly, it furnishes a means for permitting return of condensed oil to the boiler. The individual perforations in the resilient member are sufliciently small to support a film of pump oil of sufficient strength to resist a gas pressure differential on the order of one mm. Hg absolute or greater. However, the total number of perforations is sufiiciently large so 2,721,025 Ce Patented Oct. 18, 1955 that the total flow of oil through the perforated member is equal to the amount of oil entering the pumping chamber from the jet assembly.

In a preferred embodiment of the invention, the resilient perforated member comprises a thin sheet of metal in which a number of slits are cut. The slotted member is then formed into an outwardly flaring skirt which is preferably secured to the outside of the vapor jet assembly. Due to the outward flare of the slotted member, the size of the slots therein increases to a maximum at the point of maximum radius.

Referring now more particularly to Fig. l of the drawing, the pump body is illustrated at 10 in the form of a cylindrical tube which is open at its upper end 11, and within which a jet assembly 12 is positioned. In a preferred form shown, the jet assembly preferably comprises a lower cylindrical portion 14 which terminates at its top in a jet 16. This lower cylindrical portion 14 is preferably concentrically positioned with respect to the pump body and defines a pumping chamber therewith. The jet assembly 12 also includes second and third cylindrical portions 18 and 22 which terminate in second and third jets 20 and 24 respectively. A foreline 26 is connected to the pump body adjacent an opening 28 in the pump body, this foreline in operation being preferably connected to a suitable mechanical vacuum pump. Cooling coil 3% is preferably wrapped around the foreline 26 and around the outside of the pump body 10, this cooling coil maintaining the foreline and pump body sufiiciently cool so that the pump oil vapors striking either the pump body or the foreline are condensed thereon.

At the bottom of the pump body 10 there is provided a relatively large space in which a quantity of pump oil 32 may be confined. This space is designated as the boiler portion 34. The three jet tubes 14, 18 and 22 are provided with openings at their bottoms which communicate with the vapors of the pump oil 32 generated in the boiler portion 34. Connected to the base plate 36 of the pump in heat transfer relationship thereto, is a heating element 38.

Positioned between the outside of the jet tube 14 and the inside of the pump body It), and defining the bottom of the pumping chamber, is a perforated resilient member 40. As shown particularly in Figs. la and lb, this member 40 comprises a thin slotted sheet of metal attached to the outside of the jet tube 14. The slotted member 40 is preferably formed of a poor heat conductor, such as stainless steel, and is relatively thin, on the order of .005 inch, so as to limit the amount of heat passing along member 40 from the chimney 14 to the pump casing 19. This slotted member 40 is bent around the jet tube 14 and flares outwardly so as to contact the inside of the pump body 10 to accurately position the jet tube assembly concentrically with respect to the pump body.

The operation of the resilient perforated member 40 is understood best from a consideration of Fig. 1a, where the condensed oil returning to the boiler is indicated at 32a. This oil, since it is relatively cold and viscous, will Wet the surface of member 40 and cover the slots 41 with a thin film 32b of oil. A pool 32c of relatively cool oil is formed between the resilient perforated member 40 and the pump body 10. The level of the pool is selfregulating since an increased flow of oil 32a into the pool results in a somewhat deeper pool whose increased hydraulic head gives a correspondingly increased flow of oil through the narrow slots 41. Additionally, since there is a considerable temperature drop along the perforated member 40, the oil near the chimney 14 will be considerably hotter than the oil near the pump casing 10. This hot oil, due to its lower viscosity, will flow more readily through the narrow slots 41, thereby increasing the selfregulating tendency of the arrangement. Thus, the perforated member 40 is completely covered with at least a thin film of oil which prevents the vapors generated in the boiler from passing upwardly into the pumping chamber.

In the operation of the diffusion pump described above, the upper end 11 of the pump body is connected to a vacuum-tight line leading to the apparatus to be evacuated and the foreline 26 is connected to'a suitable backing pump, such as a usual mechanical vacuum pump. Coolingcoil 30 is connected to a suitable source of cooling water, and the boiler is filled with a suitable diffusion pump oil to approximately, the level indicated in the drawing. The mechanical vacuum pump is then operated to remove most of the air from the diffusion pump and the system being evacuated. The heater 58 is energized to heat the pump oil to a temperature such that its vapor pressure is on the order of 0.5 to 2 mm. Hg abs. The

' vapors generated in the pump boiler travel up the inside of the jet tubes 14, 18 and 22 and are discharged from the jets 16, and 24 respectively, with high velocities, thus pumping air from the opening 11 and forcing it out the foreline 26. The hot oil vapors condense on the inside of the surface of the cooled pump body 10 and run down this pump body until they strike the perforated member 40. The condensed oil runs to the portion adjacent the .body.wall where it is permitted to slowly pass through the thin slots to the boiler portion. Because of the small size of the slots, a quantity of cold condensed oil is retained by the perforated member 40 so as to form a permutable oil seal between the boiler 34 and the pumping chamber.

The particular arrangement described above gives a V self-regulating pool of cold oil which, together with the seal formed by the thin film of oil spanning the slots in member 40, acts to complete the barrier between the boiler and the pumping chamber, The pump is preferably so designed that the oil on the member 40 is maintained at a temperature such that its vapor pressure is less than the vapor pressure in the pumping chamber adjacent the foreline. In those cases where air cooling of the pump housing is not suflicient to accomplish this cooling of the oil, additional cooling coils can be provided adjacent member 40. The fact that the pool of oil is primarily held next to the pump housing 10 aids in maintaining this pool atrthe requisite low temperature. The low heatconductivity of the member 40 also assists in preventing an undue amount of heat transfer from the hot chimney 14 to the oil pool 320.

While one preferred type of resilient perforated member 40 has been described above, it is subject to considerable modification without departing from the scope of the invention. For example, punched holes or pierced slots can be employed in lieu of the slots particularly illustrated. Equally, the perforated member can be formed of a finemesh screen. In both such modificationathe principal requirements are that the member be resilient, to assist in centering the jet assembly, and that the individual perforations be small, so as to permit the formation of the thin film of oil over the perforation to serve as the vapor seal. A typical perforation suitable for use with dioctyl phthalate and oils of similar viscosity is preferably on the order of .010 to .025 inch.

It has been found that the resilient perforated member 40 serves several very important functions. In the first place, it provides a cheap, simple arrangement for insuring accurate centering of the jet tube assembly with respect to the pump body independent of any non-circularity of the pump body. Secondly, it prevents hot pump oil vapors from traveling upwardly from the boiler through the lower portion of the pumping chamber to the jet F regions where these vapors would cut down the speed of the pump and the ultimate vaciunn attainable with a" given pump. Thirdly, it prevents loss of oil, as the vapors from the boiler are substantially prevented from being swept out through the foreline. Fourthly, by giving im-' proved isolation of the pumping chamber and boiler, it

lengthens the period of time that the pump can be operratus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A high vacuum diffusion pump comprising a pump body, a vapor jet assembly positioned Within said pump body, said jet assembly being smaller than said pump body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged 'to discharge a jet of dilfusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing diffusion pump oil at the bottom, of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a foreline connected to said 'pump body adjacent the bottom of said pumping chamber, and a perforated resilient member positioned between said jet assembly and said pump body and defining the bottom of said pumping chamber, said perforated resilient mem ber bridging the space between the outside of said jet assembly and the inside of said pump'body so as to resiliently position the vapor jet assembly with respect to the pump body, the total number of perforations and the size of the perforations being sufficient to permit a flow body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of diflusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing diffu-' sion pump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors gener ated in said boiler, a foreline connected to said pump body adjacent the bottom of said pumping chamber, and' a perforated resilient member positioned between said jet assembly and said pump body and defining the bottom of said pumping chamber, said perforated resilient member bridging the space between the outside of said jet assembly and the inside of said pump body so as to resiliently position the vapor jet assembly with respect to the pump body, the total number of perforations and the size of the perforations of said member being suflicient to retain a quantity ofpump oil so as to form a permutable oil seal between the outside of said jet assembly and the inside of said pump body, said oil seal being thicker adjacent the pump body so as to maintain said pool of oil at a relatively low temperature.

3. A high vacuum difiusion pump comprising a pump body, a vapor jet assembly positioned within said pump body, said jet assembly being smaller than said pump body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of diffusion pump oil vapors downwardlyinto said pumping chamber, a boiler for vaporizingdiffusionpump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a foreline connected to said pump body adjacent the bottom of said pumping chamber, and a perforated resilient member positioned between said jet assembly and said pump body'and defining the bottom of said pumping chamber, said perforated resilient resiliently position the vapor jet assembly with respect to the pump body, individual perforations of said member being sufficiently small to support a film of pump oil of suflicient strength to resist a gas pressure difierential on the order of one mm. Hg absolute and greater, the total number of perforations and the size of the perforations being sufiicient to permit a flow of oil back to the boiler equal to the amount of oil entering the pumping chamber from the vapor jet assembly, the resilient member being formed of a relatively thin metal which slopes downwardly from the jet assembly to the pump body, the resilient member being a relatively poor heat conductor and serving to support a relatively thick layer of cooler oil adjacent the pump body.

4. A high vacuum diifusion pump comprising a pump body, a vapor jet assembly positioned within said pump body, said jet assembly being smaller than said pump body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of diffusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing difiusion pump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a foreline connected to said pump body adjacent the bottom of said pumping chamber, and 25 a slotted resilient member positioned between said jet assembly and said pump body and defining the bottom of said pumping chamber, said slotted resilient member bridging the space between the outside of said jet assembly and the inside of said pump body so as to resiliently position the vapor jet assembly with respect to the pump body, the slots tapering outwardly from the jet assembly, those portions of the slots closest to the jet assembly being sufliciently narrow so that the pump oil can form a continuous film across the slots even at the relatively higher temperature of this portion of the oil, the continuous film being strong enough to resist a pressure differential on the order of one mm. Hg absolute and greater, the total number of slots and the size of the slots being suflicient to permit a flow of oil back to the boiler equal to the amount of oil entering the pumping chamber from the vapor jet assembly.

References Cited in the file of this patent UNITED STATES PATENTS 2,585,139 Lawrance Feb. 12, 1952 FOREIGN PATENTS 438,437 Germany Dec. 15, 1926 

